function fearNotLetter( str ) {
  //use unicode
  const unicode = [];
  const start = str.charCodeAt( str[ 0 ] );
  for ( let i = 0; i < str.length; i++ ) {
    unicode.push( str.charCodeAt( i ) );
  }
  //check if unicode array is sequential
  for ( let n = 0; n < unicode.length; n++ ) {
    if ( ( unicode[ n + 1 ] - 1 ) != unicode[ n ] && unicode[ n ] != unicode[ unicode.length - 1 ] ) {
      //fill in the gap (unicode[n+1] and unicode[n]+1 are different!)
      return String.fromCharCode( unicode[ n ] + 1 );
    }
  } //will return 'undefined' if str is not missing any letters
}

console.log( fearNotLetter( "abce" ) );
